This invention is an improvement introduced in the system for continuous temperature measurement of molten steel using an optical process.
In the past few decades, steel mills have experienced constant improvement in terms of both productivity and energy conservation controls, as well as from the standpoint of environmental preservation.
Taking into consideration that steelmaking processes demand strict thermal control, particularly at high temperatures, temperature measurement plays a fundamental role in the accomplishment of the conditions required for the proper production flow development. Chemical reactions between gases and solid matter inside the blast furnace and the steel refining vessels are examples of process stages in which temperature control is essential to achieve the best results in terms of product quality.
In view of the great difficulty to measure the temperature during these processes, since it involves handling molten steel in the converters, ladles and continuous casting tundish, the temperature monitoring is made through the utilization of special platinum or noble metal alloy thermocouples. Such thermocouples are attached to disposable heads for eventual measurements or properly protected for long measurements, as it is the case in the continuous casting solidification process. This process requires continuous temperature measurement in order to obtain optimum continuous casting machine operating control and proper slab quality.
Nowadays, the adopted method is the optical infra-red pyrometer, whose operating principle is based on the light emitted by a certain material. The light is captured by a sensor and transmitted by optical fiber up to a signal converter (from optical to electric signal). The readings are processed by the electronic signal converter by means of a mathematical equation that calculates and displays the temperature. This system is basically made up of a measuring device, articulated manipulator, signal conversion device and a control system.
The measuring device is mechanically linked to the articulated manipulator and electrically connected to the signal conversion device.
The optical unit, which is the very measuring unit, comprises one optical sensor, fastening tube, steel hose and optical fiber. The optical fiber carries the signal from the optical sensor up to the signal conversion device, which for its turn transmits it, as an electric signal, to the control system.
The measuring tube is a ceramic tube that is dipped into the molten metal and works as a field of view for the optical sensor.
In order to take a measurement, the articulated manipulator handles the measuring system so as to position it above the tundish and moves it until the ceramic measuring tube plunges into the metal bath down to a pre-established depth. The adjustment of the distance between the optical sensor and the measuring tube end dipped into the metal bath, along with the characteristic inside profile of the measuring tube and the sensor alignment, cause the sensor to focus on the desired field of view.
From the economic point of view, this method has proved to be more advantageous than the use of type B platinum sensor (thermocouple). However, it faced certain technical and operating problems, such as: high temperature in the infra-red sensor body (>200° C.); need for monitoring the temperature to protect the sensor; type of sensor protection and installation box; stainless steel tube length and inside diameter required to fasten the sensor; to make the application suitable and the ceramic device exchange easier; ideal sensor and optical fiber cooling without affecting the measurement; accurate determination of the distance between the sensor and the bottom of the ceramic tube; definition of the ceramic tube inside diameter in order to carry out an exact measurement; suitable ceramic tube length; occurrence and elimination of silica inside the ceramic tube; gas pressure variation in the sensor and optical fiber cooling system; measurement interference when the steel bath level changes; fastening and alignment of the sensor in the stainless steel tube; adjustment of measurement response time; correction factor adjustment; measuring ratio of the two sensors (slope).